ported vacuum vs. full vacuum

[calpolymustang]

on my edelbrock 1406 600cfm, whats the difference between running off the upper vacuum connection or the lower vacuum connection?

i drive the car as a daily driver that lives stop-light to stop-light on the weekends. right now im running 15 initial, 38 total timming

 

[Ronstang]

If your talking about the vacuum advance then it needs to be attached to ported vacuum and not manifold vacuum. Ported vacuum will be 0 or very little at idle and increase as speed increases and manifold vacuum is highest at idle and cruise and decreases to 0 during acceleration......so it doesn't seem to hard to figure which vacuum signal the vacuum advance needs now does it?

 

[Hydrocarbon]

I had mine on full manifold vacuum. It seemed to run better there.

 

[bnickel]

if the car runs better with full manifold vacuum, then you either have your initial timing seet too low or the distribitor needs to be recurved. with full mainfold vacuum pulling on the advance you effectively have the vacuum advance locked out at full advance

 

[1970 slantroof]

Amen to what bnickel said. In this hot country, you will not get by long running the advance off the wrong port. When I got my 65 F-100 with a 352 it had the same symptoms and I resolved it by fixing the distributor and timing it properly.
Howard

 

[Hydrocarbon]

I've since converted to EFI. I decided to let the computer recurve the advance for me.
One thing I did notice when it was carbed. Was if it was on ported vacuum i would get a surge of power at 1500 RPMs. If it was on full vac. it was smoother.

I don't see how having it on manifold vacuum would be a bad thing as long as it doesn't ping. I thought you wanted advanced timing at idle and lite load conditions. Since my cars has 2.73's 35MPH is at about 1200RPMs, and on ported it would only start to get vacuum.
The only down side I can see, is when decelerating it would max out the advance and increase NOx emissions from the increase combustion temps.

Can someone please explain?

 

[geegee]

Ported vaccuum is obtained in the carb above the throttle butterfly whereas straight vaccuum is manifold vaccuum (below the butterfly). Ported vaccuum thus is modulated by the position of the butterfly. If the butterfly is closed, no vaccuum, etc.

 

[351LX]

I have always hooked mine to manifold as it idles better.Here is a link that describes the difference in non-technical terms. http://www.gofastforless.com/ignition/advance.htm